Paleocene sediment recovered from Blake Nose during ODP's Leg 171b shows strong color variation, from green to white. Age control from calcareous nannofossils indicates the variations occur on the order of Milankovitch precession, ~20,000 years.
Yet, when we shipboard sedimentologists examined the green and white beds using smear slides, we could find no big difference in sediment composition between the alternating sediment types. The white beds have slightly more calcite as nannofossils and foraminifers.
Shipboard magnetic susceptibility measurements indicate that iron-bearing, magnetically-susceptible minerals track the carbonate content: more mag-sus minerals in the green beds, more calcite in the white beds.
But what is actually varying between green bed and white bed deposition? What earthly sedimentologic agent changes as the Earth passes from Northern Hemisphere perihelion to Northern Hemisphere aphelion over a period of ~20,000 years?
Does carbonate productivity wax and wane? Or does carbonate remain constant and terrigenous input as mag-sus minerals wax and wane? Since these components sum to 100% of the sediment, how can you tell which varies? Or do both components co-vary?
We chose to take Arthur's "holistic" approach to the problem by analyzing each component of the sediment to find out what else varies to the precessional beat: grain size? biogenic silica content? Mineral composition of the sediment? Composition of the calcareous flora?